i was used to rebuild the whole firefox to get it in my language because simple installing it.xpi did not work.
i'm really pleased to see that in cooking there are some firefox-langpack packages. i've build the italian one modifying the firefox-langpack-fr receipt replacing fr with it in the whole file and building the receipt and installing the resulting package, so having firefox in your own language should be trivial now.
this is the italian receipt for 3.6.12

# SliTaz package receipt.

PACKAGE="firefox-langpack-it"
VERSION="3.6.12"
CATEGORY="network"
SHORT_DESC="Italian Language Package for firefox"
MAINTAINER="xxx@xxx"
DEPENDS="firefox"
BUILD_DEPENDS="unzip"
WEB_SITE="http://www.mozilla.com"
TARBALL="it.xpi"
Path=langpack-it@firefox.mozilla.org
URL="http://releases.mozilla.org/pub/mozilla.org/firefox/releases/$VERSION/linux-i686/xpi/$TARBALL"

# Rules to gen a SliTaz package suitable for Tazpkg.
genpkg_rules()
{

        mkdir -p $fs/var/$Path
        if [ -f $SOURCES_REPOSITORY/$PACKAGE-$VERSION.xpi ]; then
                unzip $SOURCES_REPOSITORY/$PACKAGE-$VERSION.xpi -d $fs/var/$Path
        else
                [ -L /usr/bin/wget ] && tazpkg get-install wget --forced
                wget -O $SOURCES_REPOSITORY/$PACKAGE-$VERSION.xpi $URL
                unzip $SOURCES_REPOSITORY/$PACKAGE-$VERSION.xpi -d $fs/var/$Path
        fi
        
}

post_install()
{
        sed -i '/pref/s/en-US/it/' /etc/firefox/pref/firefox-l10n.js
        RealPath=/usr/lib/firefox-*/extensions/    
        cp -af /var/$Path $RealPath
        rm -rf /var/$Path
}

post_remove()
{
        sed -i '/pref/s/it/en-US/' /etc/firefox/pref/firefox-l10n.js
        rm -rf "/usr/lib/firefox-*/extensions/$Path"
}

and this is the resulting file, useful for italian users:
http://dump.no/files/91ba0a56ac30/firefox-langpack-it-3.6.12.tazpkg